The field of this invention concerns electrostatic coating or spraying of substrates, and in particular, electrostatic coating or spraying of printed circuit boards with photopolymerizable solder mask.
Solder mask is a photocurable polymeric composition used to protect printed circuit boards during etching. Typically solder mask contains styrene maleic anhydride copolymers partially esterified with a hydroxy alkyl methacrylate, reactive monomers and certain multifunctional epoxides. Suitable solder mask compositions are disclosed in published European Patent Application 0 366 333, the disclosure of which is herein incorporated by reference.
Single, double, or multilayer printed circuit boards may be coated on one side or two sides with solder mask. After the board has been coated and dried, a photographic negative containing opaque images of areas where solder connections are to be made on the board is laid over the board surface. The board and negative are then exposed to ultraviolet light; the light that passes through the negative crosslinks the photopolymer in the solder mask. After exposure, the board is "developed" in a solvent system to wash away any photopolymer which was not crosslinked. The cross-linked photopolymer adhering to the board will protect the underlying material during subsequent treatment processes and when placed in actual operation. After washing and drying, the board is dipped into molten solder. Solder adheres to the board areas not covered by the mask composition. Electronic components may now be easily mounted to the board at the soldered connecting points.
U.S. Pat. No. 4,949,665, to Weber, the disclosure of which is herein incorporated by reference, discloses apparatus for coating both sides of printed circuit boards. The apparatus includes a conveyor having a moveable transport slide into which the circuit board to be coated is placed. The board thus travels through a coating device where it is coated on one side with a coating. The transport slide is then halted on the conveyor and turned around. The transport slide then travels back through the coating device so that the second side of the circuit board can be coated in the same manner in which the first side was coated. The board is then transported into a dryer where both sides of the board are dried uniformly.
One conventional means for applying the solder mask to circuit boards is by electrostatic spraying. Apparatus employing an electrostatic field is disclosed in U.S. Pat. No. 4,826,703 to Kisler, of which is herein incorporated by reference. An electrically conductive nozzle is used as the coating applicator, to which the coating material is pumped at a desirable flow rate. An electrostatic field is created so as to charge the coating particles. The web substrate is oppositely charged. The '703 patent also discloses controlling the width and improving the edge thickness uniformity of a layer of coating material that is electrostatically deposited on a moving web surface. This is accomplished by providing a pair of electrodes coupled to a source of electrical power and mounted in an opposed relation on opposite sides of the charged coating material moving between the coating applicator and the web surface spaced therefrom.
The use of electrodes to direct the coating particles to the substrate being coated is especially advantageous when using rotary atomizer nozzles to spray the coating. The nature of rotary atomizers is such that absent means to confine the spray pattern of the coating, the coating tends to be propelled in a wide pattern, thereby coating more than the circuit board. This results in considerable downtime, as the apparatus must be cleaned of the coating (solder mask). In addition, the coverage on the substrate tends to be non-uniform and in the shape of a raised donut; that is, the center portion of the substrate tends to have less coating than the outer circumference.
A further problem in electrostatically coating circuit boards occurs as one circuit board passes past the spray area. The next circuit board to be coated is spaced from the board previously coated, thereby creating a void into which the coating material is being sprayed while the second board is traveling towards the spray area. Since the boards attract the charged coating particles, as the first board passes by the spray area and the second board approaches the spray area, the coating particles do not pass straight through the gap between the boards, but rather tend to "wrap around" the boards and deposit on the side of the boards not facing the nozzle. This results in a non-uniform coating on the opposite sides of the boards and is highly undesirable.